1. Field of the Invention
The invention relates to a steering angle sensor system and a method for measuring the steering angle in a motor vehicle.
2. Description of the Related Art
Methods for measuring angles or angle measuring devices are frequently used to sense the steering angle in motor vehicles in order, for example, to determine the steering angle in vehicle movement dynamics controllers (ESP). Furthermore, it is also necessary to sense the steering angle for motor vehicle control systems which carry out an active steering intervention.
In this context it is also known that particular requirements are made of the availability of the steering angle measurement through the possibility of active steering interventions by a motor vehicle control system. A specific requirement which such a motor vehicle control system makes of the angle measuring device during the time directly after the activation of the motor vehicle is to know the steering angle even though steering movements or indirect steering movements, caused by external interference influences, may even occur when the vehicle is not operating, and they have to be taken into account.
Document EP 1 238 891 B1 discloses a steering angle sensor which, in addition to a coding disk whose position, and therefore the absolute angle, within a revolution is sensed by means of sensors, has a mechanically coupled counting wheel for sensing the number of revolutions. The absolute angle is calculated from a combination of the output signals of two magnetic field sensors and the position of the counting wheel. In order to be able to carry out an absolute angle measurement using this sensor directly after the motor vehicle starts, the steering angle sensor has to be supplied with voltage even during an inactive state of the vehicle, as a result of which, depending on the duration of this state, the electrical power supply of the motor vehicle is heavily loaded. Furthermore, the mechanism is relatively complex and subject to wear.
In document EP 0 874 751 B1 it is proposed to use a mechanical counting unit in addition to a rotational angle sensor which measures the absolute angle within a revolution with the aid of a coding disk. In this context, the mechanical counting unit is mechanically coupled to the coding disk and counts the revolutions. The absolute rotational angle at each time is also sensed over more than one revolution. A disadvantage with this design is the extremely complex mechanism of the coding disk and of the counting unit, as well as the transmission of torque by the mechanical coupling, which is subject to wear and does not function quietly.
Document DE 103 44 267 A1 describes a method which uses a semi-relative steering angle sensor. Here, in the state in which the ignition of the vehicle is switched off steering wheel movements are sensed by the steering angle sensor and stored electronically. When the ignition is activated, and there were no faults to be registered, these data are used to calculate the current steering angle.
This system is disadvantageous insofar as here too both the sensor and the electronic memory have to be supplied with power in the inactive state of the vehicle, which proves a drain on the vehicle battery. Furthermore, in the event of faults the angle is re-standardized and cannot be sensed directly by the steering angle sensor. The standardization is carried out by detecting straight-ahead running during travel, in which case this straight-ahead running is assessed on the basis of the profile of the yaw rate over time.